Mark RetryException#getCause non null

This commit simplifies RetryException to always require a root cause
and mark it as not nullable. Such exception is the exception thrown by
the retryable operation and should always be available as it explains
why the invocation was a candidate for retrying in the first place.

Closes gh-35332

Author: snicoll

spring-context/src/main/java/org/springframework/resilience/retry/AbstractRetryInterceptor.java

@@ -112,8 +112,7 @@ public String getName() {
 			});
 		}
 		catch (RetryException ex) {
-			Throwable cause = ex.getCause();
-			throw (cause != null ? cause : new IllegalStateException(ex.getMessage(), ex));
+			throw ex.getCause();
 		}
 	}
 

spring-core/src/main/java/org/springframework/core/retry/RetryException.java

@@ -17,6 +17,9 @@
 package org.springframework.core.retry;
 
 import java.io.Serial;
+import java.util.Objects;
+
+import org.jspecify.annotations.NonNull;
 
 /**
  * Exception thrown when a {@link RetryPolicy} has been exhausted.
@@ -31,14 +34,6 @@ public class RetryException extends Exception {
 	private static final long serialVersionUID = 5439915454935047936L;
 
 
-	/**
-	 * Create a new {@code RetryException} for the supplied message.
-	 * @param message the detail message
-	 */
-	public RetryException(String message) {
-		super(message);
-	}
-
 	/**
 	 * Create a new {@code RetryException} for the supplied message and cause.
 	 * @param message the detail message
@@ -48,4 +43,10 @@ public RetryException(String message, Throwable cause) {
 		super(message, cause);
 	}
 
+
+	@Override
+	public synchronized @NonNull Throwable getCause() {
+		return Objects.requireNonNull(super.getCause());
+	}
+
 }